Vulin,
I think it's a pretty straight deal to use historic or vinatge plates...I am in New York and this is what I ran up against...
I own a 1972 Int'l Scout II...I found some really cool vintage plates for it....there was not a problem to use them, provided I sent a high quality scan or photo of them to the DMV along with the historic/vintage plate application, the plates themselves were clear in the books of the DMV and the insurance card states that the vehicle is insured as a historic/vintage vehicle. ALSO, i found out while speaking with my insurance company the vehicle, to be insured as historic/vintage, can't be your primary vehicle...
I am not sure if the same rules apply in PA, but it couldn't be far off. probably in the case of the bike being the second vehicle it'll all turn out fine.
I started getting too curious about your situation and came up with this....PA DMV form MV-11
APPLICATION FOR PERMANENT,
ANTIQUE, CLASSIC OR
COLLECTIBLE REGISTRATION
PLATE
[Broken Link Removed]
good luck....
Eric